Infanticide in Mammals

I was very excited to see this year’s Disney Nature film, “Bears.” The movie follows a first time brown bear (Ursus arctos; note in North America the brown bear subspecies is the grizzly bear Ursus arctos horribilus) mother and her two cubs from their den exit to next year’s entrance. Bear cubs are born in dens and spend the first few months nursing while their mother hibernates. The first bear fact the movie presented was that 50% of cubs of the year do not survive, and in this way the movie prepared the audience that one of the two cubs may die during the movie.

While I really did enjoy this movie, I had a quibble with the narrative. The filmmakers showed multiple scenes where large males advanced on the female and her cubs. They narrated these scenes as the males wanting to eat the cubs as a source of food. While eating young in resource scarce environments has been reported, biologists hypothesize that male aggression on un-weaned young is more related to mating and reproductive fitness.

Infanticide, the killing of young by adults of the same species, has been reported in numerous mammals including bears, leopards, gorillas, and dolphins just to name a few. Both adult females and males have been reported to commit infanticide, although the prevalence by one sex may vary with species. For example, this paper reports the risk of female infanticide in mongooses, a cooperative breeding species. Infanticide by males has been widely reported; for brown bears, 80% of infanticidal attacks are by males. Attacks may be sex biased with adult males killing male offspring.

There are several hypotheses about infanticide; however, the prevailing hypothesis is that it is a reproductive strategy for males. If a female loses the young that she is caring for, then she will go into oestrus sooner than if her young survive and she cares for them until they disperse as juveniles (for bears this usually occurs when they are 2). If the female losses her young early enough, she may be capable of mating in the current breeding season. This may provide the male an opportunity to mate with the female. In this way the male perpetrating infanticide, can increase his reproductive fitness. Infanticide decreases female fitness, so females have evolved multiple behaviors to avoid infanticide; two were shown in “Bears” including avoiding feeding places with dominant males and defending offspring including directly fighting with attacking males.

There’s a lot more literature on infanticide including the evolution of social structure (and here).

Overall, I thought “Bears” was a fun and engaging movie that showcased the beautiful Alaskan wilderness. I just wanted to point out that the attacks on the cubs were more than a big bear in need of a snack.

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